Douglas used to do it all the time - sometimes at 4 o clock in the morning!
At first I got a bit freaked out by it too, but then I realised that it was little things like the wind, or a car door shutting outside, or someone walking past the house, or the water pipes gurgling, that was setting him off.
I started asking him "what is it", encouraging him to make sure for himself that there wasnt anything wrong, and eventually he learnt to trot off, go and have a nose round, decide that everything was ok, and then settle back down again!!!

Sounds like your dog is going to make a great guard dog!!!

Tams x